In general, a fluid flow control device is mounted in a fluid regulator, such as valves, in order to properly control kinetic energy of fluid flowing therethrough. Ultimately, the fluid flow control device is to control pressure, velocity, flow rate, noise, cavitation, and so on of the fluid into a proper level. The kinetic energy of the fluid is determined according to the flowing velocity of the fluid. Furthermore, the flowing velocity of the fluid is directly associated with a local loss (local fluid resistance) determined by a pressure difference (differential pressure) of the fluid acting between an inlet and an outlet of the fluid flow control device, density of the fluid, forms of fluid pathways, and the Reynolds number.
In other words, the flowing velocity of the fluid in the fluid flow control device is in inverse proportion to the square root of a loss coefficient of a fluid resistance part under a specific differential pressure condition applied to the device or the fluid resistance part, and the kinetic energy of the fluid is proportional to the square of the flowing velocity. Thus, in order to lower the kinetic energy of the fluid into a proper level, the loss coefficient of the fluid resistance part must be increased. Based on the theory of fluid dynamics, the flowing velocity (V), the kinetic energy (KE) and the flow rate (w) will be expressed as follows.
                    V        =                  w                                    ρ              0                        ⁢                          A              0                                                          [                  Mathematical          ⁢                                          ⁢          formula          ⁢                                          ⁢          1                ]                                KE        =                              1            2                    ⁢                      ρ            0                    ⁢                      V            2                                              [                  Mathematical          ⁢                                          ⁢          formula          ⁢                                          ⁢          2                ]                                V        =                              (                                          2                ⁢                Δ                ⁢                                                                  ⁢                P                                                              ρ                  0                                ⁢                ζ                                      )                                1            2                                              [                  Mathematical          ⁢                                          ⁢          formula          ⁢                                          ⁢          3                ]            
Wherein, ΔP means a differential pressure acting to the device or the fluid resistance part, p0 means density of the fluid, A0 means a cross-sectional area, ζ means a loss coefficient of the fluid resistance part, and V means the flowing velocity of the fluid.
Finally, in order to lower the kinetic energy of the fluid acting to the fluid flow control device under the specific differential pressure condition, the total local fluid resistance must be increased. Moreover, in order to increase a flow rate of the fluid in a state where the kinetic energy of the fluid is controlled properly, the total sectional area of the flow path must be increased. Through the above-mentioned properties, it is possible to make the fluid flow control device small-sized.
In the meantime, if the kinetic energy of an outlet of the fluid flow control device is less than 480 kPa (corresponding to 30 m/s of water) under general flowing conditions, there are few side effects of noise, vibration and Flow-Accelerated Corrosion by the fluid. However, if the kinetic energy is more than 1,030 kPa, it may cause severe damages on the fluid flow control device and apparatuses on which the fluid flow control device is mounted. Particularly, under conditions that cavitation may be caused or under conditions of two-phase fluid, the kinetic energy must be limited to 275 kPa (corresponding to 23 m/s of water) or less. Additionally, in case of systems sensitive to vibration, it is suggested that the kinetic energy must be limited to 75 kPa (corresponding to 12 m/s of water) or less.
Furthermore, the fluid flow control device must also give consideration of noise, and in this case, the primary noise source is aerodynamic noise. The level of noise energy is related with a mass flow rate, a pressure ratio of the absolute pressure of the upstream side to the absolute pressure of the downstream side, a geometrical structure, and physical properties of fluid. Because an increase of the pressure ratio at a specific area causes a sound velocity flow or a choke flow, which is a high noise source, the pressure ratio must be controlled to thereby reduce or prevent the incidence of noise.
Accordingly, it is necessary to keep a proper flowing velocity by keeping the kinetic energy of fluid to the standard level or below and preventing a sudden change in pressure of the fluid. Various prior arts to satisfy the need have been disclosed. Particularly, various kinds of fluid flow control devices based on a cylinder shape, as shown in the present invention have been disclosed. Such fluid flow control devices can be divided by a method of changing the size of cross-sectional areas of fluid pathways and a method of suddenly changing directions of the fluid pathways.
In relation with the method of changing the size of cross-sectional areas of the fluid pathways, there are U.S. Pat. No. 4,921,014 registered on May 1, 1990, U.S. Pat. No. 5,018,703 registered on May 28, 1991, Korean Patent No. 0280893 registered on Nov. 13, 2000, U.S. Pat. No. 6,394,134 B1 registered on May 28, 2002, and U.S. Pat. No. 6,766,826 B registered on Jul. 27, 2004.
A fluid flow control device disclosed in Korean Patent No. 0280893 and U.S. Pat. No. 6,394,134 B1 will be described as below. The fluid flow control device includes: an inside cylinder having a plurality of holes and protrusions formed uniformly in axial and radial directions; an outside cylinder having a plurality of holes formed uniformly in axial and radial directions; and first internal cylinders being overlapped and inserted between the inside and outside cylinders and forming a plurality of holes on a plurality of grooves forming rectangular section elbows in the axial direction. Moreover, the fluid flow control device further includes a cage having second internal cylinders which have a plurality of holes formed uniformly in axial and radial directions and combines between the first internal cylinders, and disk-like upper and lower supporting plates for coupling the inside and outside cylinders with the first and second internal cylinders. Thus, a flowing path of the fluid is divided by the axial direction, and each fluid path has grooves forming an orifice, a rectangular section elbow and a recess so as to control a flow of the fluid. However, the invention according to prior art adopts a method of causing a sudden change of a flow path cross-sectional area, such as the orifice, at a portion where a local loss is induced. Thus, it locally causes an increase of flow velocity of the fluid and a sudden change of pressure, and hence, it may cause noise, vibration, Flow-Accelerated Corrosion, and so on and deteriorate performance of the device due to foreign substances stuck on a portion that the cross-sectional area becomes narrowed.
In U.S. Pat. No. 6,766,826 B2, a cage which is a fluid flow control device includes one or more windows and one valve seat. Additionally, each of the one or more windows includes a plurality of slots each having a longitudinal axis. The longitudinal axis of each of the slots is parallel to or tilted at a relatively small angle with respect to a reference plane that is perpendicular to a cage bore axis. The slots are tapered in width through the wall of said cage, and preferably, increase in width with increasing radius through the wall of said cage. In case that the slots are applied for control of liquid, the slots accelerate the flow when the liquid flowing direction is oriented toward the cage bore axis to thereby reach the highest velocity at the inner surface of the cylinder. This may cause the static pressure at that point to reach the vapor pressure of the liquid and consequently force partial vaporization of the jets. This vapor then collapses into liquid flow within cage bore. The invention of prior art decreases noise, but since the pressure ratio of the upstream pressure of the slots to the downstream pressure of the slots is excessive, noise is still generated under specific driving conditions. Furthermore, since the prior art artificially generates cavitation by accelerating the kinetic energy of the fluid, noise and vibration are caused under the specific driving conditions, and thus, it may cause cavitation damage in the device and adjacent structures.
As an example of the method of suddenly changing the direction of the fluid pathway, there is Korean Patent No. 0436634. In Korean Patent No. 0436634, the fluid flow control device includes a plurality of communication holes independently disposed in a cage embedded in a valve body chamber mounted between a fluid inlet and a fluid outlet, and pathways in which the communication holes are respectively refracted inside a cylindrical surface forming the same axis as an axis of the cage. Furthermore, the cage is constructed in such a way that a plurality of cylindrical bodies respectively having refraction holes on the surface thereof are bonded to thereby provide great refraction to the fluid pathways, whereby the cage can effectively absorb energies of high pressure fluid and reduce noise or cavitation. However, each cylinder must have small round through-holes formed on inner and outer surfaces thereof and refraction holes or through holes formed along the circumferential surface thereof, which are communicatingly connected. Thus, in consideration of a restricted volume of the cage, a flow rate of the fluid is reduced. Accordingly, in order to control the kinetic energy and the flow rate of the fluid to a proper level, the size of the device is relatively increased, and hence, an occupation space of the device is also increased and it costs a great deal.
Moreover, in Korean Patent No. 0527918, which has been invented by the same inventor as the present invention, diagonal fluid pathways formed in a horizontal direction are all inclined at the same angle and in the same direction. In this case, when the fluid flows from the outside to the inside of the fluid flow control device, vortex occurs inside the fluid flow control device, and it causes non-uniform flowing and pressure distribution. Thus, it may cause cavitation, flashing or hammering.